Unfortunately the garage slab was not really designed for this kind of load but...
What the concrete slab rests on is important; if properly set on compacted earth there should be little problem because even soft clay is good for 2000 pounds per square foot.; gravel is good for 5000 ppsf. You are putting "only" 350 lbs per square foot on the concrete The reinforced concrete is not going to compress or separate but it may crack if the sub-grade was improperly prepared and the slab is forced to flex. With the machine being close to the wall and I assume a trench footing the load is going to be further spread which will help.
I'd put a mill in my garage and not worry about it; the worst case scenario is a hairline crack in the slab unless the builder was a real sleaze. Further, if a crack developed I'd rent a concrete saw and cut a score around the machine and let it crack all it wants. Of course your folks may not see it this way.
